Spray pyrolysis facilitated construction of carbon nanotube-embedded hollow CoFe electrocatalysts demonstrating excellent durability and activity for the oxygen reduction reaction

Abstract

Developments in fuel cell technology have led to the design of new efficient electrocatalysts for the oxygen reduction reaction (ORR). In particular, metal-nitrogen-carbon (M-N-C) catalysts are promising alternatives to conventional noble metal ORR catalysts. Since the M-N-C catalyst has lower intrinsic activity than the noble metal catalyst, M-N-C catalysts should feature large surface areas, accessibility of active sites, short mass/charge transfer lengths, and surface permeability; these properties can lead to excellent catalytic activities. In addition, carbon-based materials, such as carbon nanotubes (CNTs), can be used to effectively improve the conductivity and stability of M-N-C catalysts. However, synthesizing carbon-based metal electrocatalysts with homogeneously distributed active sites and well-controlled structures remains challenging. In this study, we designed a carbon nanotube-encapsulated hollow Co-Fe-NC electrocatalyst by combining ultrasonic spray pyrolysis and a pseudomorphic transformation to a metal-organic framework, followed by carbonization. Spray pyrolysis formed hollow spheres without additional templates, requiring no further acidic leaching steps. Pseudomorphic transformation strategies can preserve the original morphology even after transforming the material to a metal-organic framework. The prepared CoFe-NC-CNT electrocatalyst exhibited high catalytic activity (half-wave potential of 0.924 V) and excellent 4-electron ORR selectivity.